OPT01

String[] values = new String[]{"---", ".--.", "-", "..", "...-", ".", ".-."};
    Map<String, Character> map = new HashMap<>();
    List<String> ans = new ArrayList<>();

    public List<String> decode(String code) {
        for (int i = 0; i < "OPTIVER".length(); i++) {
            map.put(values[i], "OPTIVER".charAt(i));
        }
        ans.clear();
        backtrack(0, "", code);
        return ans;

    }

    private void backtrack(int pos, String res, String code) {
        if (pos == code.length()) {
            ans.add(res);
            return;
        }
        int mx = Math.min(pos + 4, code.length());
        for (int i = pos + 1; i <= mx; i++) {
            String key = code.substring(pos, i);
            if (map.containsKey(key)) {
                backtrack(i, res + map.get(key), code);
            }
        }
    }